Wavelength compensation in a WSXC using off-voltage control

ABSTRACT

A tunable liquid crystal switch is disclosed. An electronic controller provides an electronic drive scheme for achieving low intra-channel crosstalk of less than −40 dB using only electronic compensation. A cross-talk less than −50 dB is provided by combining coarse temperature tuning and electronic compensation. This is acheived by designing the thickness of the liquid crystal device to cause a minimum to occur at a wavelength longer than a longest operating wavelength and at a temperature greater than a maximum operating temperature. This ensures that the liquid crystal device is tunable over all operating wavelengths and temperatures using electronic compensation.

This application claims the benefit of priority under 35 U.S.C. §119(e)for U.S. Provisional Patent Application Serial No. 60/129,798 filed onApr. 16, 1999, the content of which is relied upon and incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical switches, andparticularly to liquid crystal optical switches.

2. Technical Background

Twisted (TN) and Supertwist (STN) nematic liquid crystal devices arewell known and can be found in numerous applications. The most prevalentuse of TN/STN devices is in the area of displays; however, these deviceshave been proposed for optical communications applications.

The nematic liquid crystal cells include alignment layers that cause theliquid crystal molecules to form a 90° helix. The helix functions as awaveguide. When no voltage is applied a polarized light signal isrotated by approximately 90° by adiabatic following. When power isapplied to the cell, the helical alignment of the liquid crystalmolecules is disrupted and the polarized light signal passes through thecell without being rotated.

In order for optical switches or wavelength selective cross-connectdevices to be feasible, they must exhibit low intra-channel crosstalk.The maximum amount of cross-talk that can be tolerated is about −35 dB.System designers are specifying systems having cross-talk that is lessthan −40 dB. The measure of cross-talk in an LC cell sandwiched betweenparallel polarizing plates is the transmissivity.

The transmissivity and its reciprocal, the extinction ratio, is directlyrelated to the degree of rotation provided by the helix in the LC cell.An LC cell that perfectly rotates a polarized light signal by 90° wouldhave zero transmissivity and an infinite extinction ratio. This is knownas the “minimum condition” and will be discussed in more detail below.However, for all practical purposes, cells having a perfect 90° helix donot exist. Thus, as discussed above, LC cells rotate a polarized lightsignal an amount approximately equal to 90°. When an orthogonallypolarized light signal, for example, is rotated by the LC cell, themajority of the signal becomes a parallel polarized signal. However,because the rotation isn't a perfect 90°, an orthogonal componentremains. The orthogonal component is transmitted by the cell and isintra-channel cross-talk in the communications channel. Obviously, theresults are similar when the input signal is a parallel polarizedsignal; a parallel component will remain.

In one approach that has been considered, a wedged-shaped nematic liquidcrystal cell was employed in an optical switch. The switch included awedge-shaped cell which was disposed perpendicular to the light beam.During use, the effective thickness of the cell was varied to obtain aminimum condition by sliding the wedge-shaped cell along a directionperpendicular to the beam. This approach has serious drawbacks. Sincethe minimum condition (see equation (2) below) is wavelength dependent,a different thickness is required for the minimum condition for eachwavelength channel. The task of designing a multi-cell liquid crystalarray wherein each cell has a different thickness or wedge shape isimpractical. This is exacerbated by the need for a different mechanicalactuator for each cell. Because of these factors, this design is limitedto a few wavelengths at most. Another drawback is reliability of thedesign. It uses mechanical movement of the cell for tuning a cell intominimum condition. Moving parts experience fatigue and ultimately fail.

Thus, a need exists for an optical switch or wavelength selectivecross-connect (WSXC) having an array of LC cells that are individuallyand dynamically tunable to provide an acceptable level of cross-talk. Aneed exists for an LC device that can be tuned without moving the cellsor the light beam itself. In addition, a need exists for a device thatcan support many wavelength channels.

SUMMARY OF THE INVENTION

The present invention addresses the needs discussed above. A tunableliquid crystal switch that achieves an intra-channel crosstalk of lessthan −40 dB using only electronic compensation is disclosed. Across-talk less than −50 dB is provided by combining coarse temperaturetuning and electronic compensation. This is acheived by designing thethickness of the liquid crystal device to cause a minimum condition tooccur at a maximum operating wavelength and a maximum temperature. Thus,the liquid crystal device is tunable over all operating wavelengths andtemperatures using electronic compensation.

One aspect of the present invention is an optical device for directing alight signal. The optical device includes a liquid crystal element formodulating the light signal, wherein the liquid crystal element ischaracterized by an extinction ratio when in an off-state. The opticaldevice also includes a voltage controller connected to the liquidcrystal element, wherein the voltage controller supplies a bias voltageto the liquid crystal element in the off-state to drive the extinctionratio toward a minimum condition.

In another aspect, the present invention includes a method of directinga light signal in an optical device. The optical device includes aliquid crystal element for switching the light signal. The liquidcrystal element is characterized by an extinction ratio when in anoff-state. The method includes the steps of providing a voltagecontroller connected to the liquid crystal element to supply the liquidcrystal element with bias voltages, and the step of supplying a biasvoltage to the liquid crystal element in the off-state to drive theextinction ratio toward a minimum condition.

In another aspect, the present invention includes a method offabricating an optical device for directing a light signal. The methodincludes the step of providing a liquid crystal element for switchingthe light signal, the liquid crystal element having a thickness “d”which causes a minimum condition of an extinction ratio to occur at alongest operating wavelength of the light signal and at a temperaturegreater than a maximum operating temperature of the optical device. Themethod also includes the step of providing a voltage controllerconnected to the liquid crystal element, wherein the voltage controllersupplies a bias voltage to the liquid crystal element in the off-stateto drive the extinction ratio toward the minimum condition for operatingwavelengths shorter than the longest operating wavelength, and fortemperatures lower than the maximum operating temperature.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the tunable liquid crystal switch inaccordance with the first embodiment of the present invention;

FIG. 2 is a plot of the extinction ratio versus the thickness of theliquid crystal cell illustrating the minimum conditions;

FIG. 3 is a plot of cross-talk versus voltage illustrating theimportance of selecting a thickness of the LC cell for a minimumcondition;

FIG. 4 is a plot of extinction ratio versus temperature illustrating theeffects of temperature variation;

FIG. 5 is a plot of the bias voltages required to provide a −40 dBextinction ratio over a range of temperatures;

FIG. 6 is a plot of the extinction ratio when using voltage compensationfor a range of wavelengths for various temperatures;

FIG. 7 is a plot of voltage versus wavelength for maintaining −40 dBextinction ratio at 54 Deg. C.°;

FIG. 8 is a plot of voltage versus wavelength for maintaining −40 dBextinction ratio at 57 Deg. C.°;

FIG. 9 is a plot of voltage versus wavelength for maintaining −40 dBextinction ratio at 59 Deg. C.°;

FIG. 10 is a schematic view depicting the electronic controller of thetunable liquid crystal switch in accordance with the first embodiment ofthe present invention;

FIG. 11 is a schematic of a WSXC according to the first embodiment ofthe present invention; and

FIG. 12 is a schematic view of the optical monitor channel in accordancewith a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.An exemplary embodiment of the first of the present invention is shownin FIG. 1, and is designated generally throughout by reference numeral10.

In accordance with the invention, the present invention for a tunableliquid crystal switch 10 includes electronic controller 30 whichprovides an electronic drive scheme for achieving low intra-channelcrosstalk less than −40 dB using only electronic compensation. Across-talk less than −50 dB is provided by combining coarse temperaturetuning and electronic compensation. An optical monitor is also providedto monitor intra-channel cross-talk to maintain low intra-channelcrosstalk.

As embodied herein and depicted in FIG. 1, a schematic view of tunableliquid crystal switch 10 is disclosed. Switch 10 includes Liquid Crystal(LC) cell 20 connected to electronic controller 30. One salient featureof the present invention relates to the design of LC cell 20. LC cell 20includes glass sheets 202 and 212 which are used for both their lighttransmission characteristics and as a casing for the components of thecell. Electrodes 204 and 210 are formed on the interior of glass sheets202 and 212, respectively. Dielectric alignment layers 206 and 208 aredisposed on electrodes 204 and 206, respectively. Nematic liquid crystalmaterial 200 is disposed between layers 206 and 208. Electrodes 204 and210 are electrically connected to electronic controller 30, as well. Oneof ordinary skill in the art will recognize that the surfaces ofalignment layers 206 and 208 cause the liquid crystal molecules to alignand form a helix having a twist of approximately 90°. As discussed inthe Background of the Invention above, the helix performs a waveguidingfunction. The thickness “d” of the liquid crystal material 200 disposedbetween alignment layers 206 and 208 is of great importance in thedesign of the present invention.

There are situations wherein the parameters of LC cell 20 can bemanipulated to obtain a minimum condition wherein theoretically, theextinction ratio can approach infinity. For a 90° TN LC cell sandwichedbetween a parallel polarizer, the transmissivity is given by:$\begin{matrix}{T = {\frac{I_{out}}{I_{in}} = \frac{\sin^{2}\left\lbrack {\frac{\pi}{2}\sqrt{1 + u^{2}}} \right\rbrack}{1 + u^{2}}}} & (1)\end{matrix}$

wherein the factor u is given by: $\begin{matrix}{u = {2\frac{d}{\lambda}\Delta \quad n}} & (2)\end{matrix}$

where d is the cell thickness shown in FIG. 1, λ is the wavelength, andΔn is the LC birefringence. From equation 1, it is clear that thetransmissivity becomes smaller as u increases. However, it only assumesa zero value under discrete conditions that satisfy: $\begin{matrix}{{\left\lbrack {\frac{\pi}{2}\sqrt{1 + u^{2}}} \right\rbrack = {m\quad \pi}},\quad {{{wherein}\quad m} = 1},2,{3\ldots}} & (3)\end{matrix}$

As embodied herein and depicted in FIG. 2, a plot of the extinctionratio versus the thickness of the liquid crystal cell illustrates theimportance of proper LC cell design. As shown in FIG. 2, the extinctionratio for minimum conditions 90 can, in practice, range between 50 dB toover 90 dB. Minimum conditions 90 occur when equation (3) is true. Thus,if LC cell 20 can be maintained in a minimum condition, or thereabout,the low crosstalk (low transmissivity-high extinction ratio) needed inliquid crystal switch 10 will be achieved. Several barriers exist tobuilding and maintaining LC cell 20 at the minimum condition. First, themanufacturing process for the cell thickness is not precise. Second, thebirefringence, An, is highly dependent on temperature. Third, eachwavelength/channel in a multi-wavelength system must be tunedseparately. If LC cell 20 is designed properly, these barriers can beovercome via electronics.

For the electronic compensation to work, LC cell 20 must be designedwith an appropriate thickness “d,” such that a minimum condition for thelongest operating wavelength occurs at a temperature just above themaximum operating temperature. This is very important to ensure voltagetunability over all wavelength and operating temperatures. Note that onecould design the LC cell 20 to the first, second or any minimumcondition and achieve similar results. Then, each channel in the “off”state, an ac signal with a RMS voltage, V_(off,RMS) (λ_(n), T), can beapplied to tune the cell to the minimum condition for its wavelength andtemperature. The type of ac waveform is not critical since the LC cellresponds to the RMS value of the signal. When a small ac signal isapplied to the cell, the LC molecules tilt slightly without destroyingthe “adiabatic following” of the 90° twist. The effect of the tilt is asmaller birefringence, Δn. Thus, a small voltage can be used to tuneinto the minimum condition.

For each pixel/channel in the “on” state, an ac signal with a large RMSvoltage V_(on,rms) (≈10 V_(RMS)) is applied, since this voltage is not afunction of channel wavelength or temperature.

As embodied herein and depicted in FIG. 3, a plot of cross-talk versusvoltage illustrating the importance of selecting a proper thickness forLC cell 20 is disclosed. Plot line 200 (1570 nm signal) illustrates animproperly designed LC cell. Because its thickness wasn't selected tocause a minimum condition to occur for the longest operating wavelengthat a maximum temperature value just above the maximum operatingtemperature, no amount of electronic compensation will drive the cellinto a minimum condition. Plot lines 202 (1530 nm signal) and 204 (1550nm signal) illustrate the benefits of a properly designed LC cell 20.Plot line 202 is driven to a minimum condition at about 1.4 Vrms andplot line 204 is driven to a minimum condition at about 0.9 Vrms. FIG. 3also illustrates the wavelength dependence of V_(off,RMS) (λ_(n), T).

As embodied herein and depicted in FIGS. 4 and 5, measurements on LCcell 20 using only electrical compensation via amplitude modulation aredisclosed. FIG. 4 is a plot of the peak extinction ratio in dBs (versustemperature at various wavelengths for LC cell 20 in which the thicknesswas chosen to give a first minimum at approximately 80° C. FIG. 5provides a plot of the optimal “off” voltage as a function oftemperature for various wavelengths required to maintain the −40 dBextiction ratio. Note that the RMS voltage must be accurate to within±10 mV.

To obtain an extinction ratio that is greater than −40 dB, coarsetemperature regulation is combined with the above discussed electricalcompensation. In the present invention, the temperature is regulatedover a limited region of approximately 6° C. and the thickness of the LCcell is selected to cause the minimum condition at the longest operatingwavelength at the largest control temperature value. Again, this isimportant to ensure voltage tunability over all operating wavelengths.

As embodied herein and depicted in FIGS. 6-9, measurements of LC cell 20using electrical compensation and thermal regulation are disclosed.Thus, performance improves when electrical compensation is performed inoptimal temperature ranges.

FIG. 6 is a plot of extinction ratio versus wavelength at varioustemperatures. The peak optimal temperature is approximately 61° C. Asshown in FIG. 6, the extinction ratio drops drastically for the longerwavelengths at 63° C., because electrical compensation becomesineffective.

FIGS. 7-9 are high-low plots illustrating the voltage ranges required tomaintain −40 dB extinction for 54° C., 57° C., and 50° C., respectively.In FIG. 7, the operating temperature is 54° C. The accuracy of theapplied waveform must be ± 10 mV. In FIG. 8, the temperature isincreased to 57° C. Note that the voltage range increrases as wavelengthincreases. In FIG. 9, the temperature is 59° C . As the wavelengthincreases to 1570 nm, the accuracy of the bias voltage is not a criticalissue and as long as it is within 0.5 Volts a high extinction ratio (>40dB) is maintained.

As embodied herein and depicted in FIG. 10, tunable liquid crystalswitch 10 includes liquid crystal device 200 connected to electroniccontroller 30. Electronic controller 30 is connected to temperaturecompensation module 34 which maintains LC device 200 at a predeterminedambient temperature. Electronic controller 30 is connected to a networkinterface. The network interface is not part of the present invention,but one of ordinary skill in the art will recognize that switch 10receives switching and configuration information from a network via thenetwork interface.

Electronic controller 30 includes temperature sensor 32 which isconnected to LC device 200 and measures the ambient temperature of theoperating environment of switch 10. Sensor 32 is connected to analog todigital (AID) convertor 302. Analog temperature values are latched intoA/D convertor 302 and converted into a digital word. One of ordinaryskill in the art will recognize that these two functions can be combinedinto a single device. Also, these functions can also be incorporated inprocessor 300, as an on-chip function. A/D convertor 302 is connected toprocessor 300 supplying it with temperature information in a digitalform. LUT 304 is connected to processor 300 and voltage modulator 306.The heart of controller 30 is processor 300 which controls the operationof A/D convertor 302, LUT 304, and voltage modulator 306 using systembus 308 which provides data, address, and control lines, along with asystem clock signal. Processor 300 also controls temperaturecompensation module 34. Temperature compensation module 34 adjusts theoperating temperature of liquid crystal device 200.

Look-up-table (LUT) 304 may be of any suitable well known type, butthere is shown by way of example a random access memory (RAM). One ofordinary skill in the art will recognize that any type of digital memorymay be used depending upon the amount of flexibility that is desired.For example, if an EEPROM is used, the V_(off,RMS) (λ_(n), T) values canbe updated by retrieving the EEPROM from switch 10, erasing thecontents, and reprogramming it with new values. A RAM on the other hand,can be repopulated dynamically. Processor 300 would merely write newvalues to each location in LUT 304. One of ordinary skill in the artwill also recognize that LUT 304 need not be used at all. In analternate embodiment, processor 300 can compute the V_(off,RMS) (λ_(n),T) values and directly control voltage modulator 306.

Processor 300 may be of any suitable well-known type, but there is shownby of example a 32-bit floating point embedded microprocessor equippedwith a real-time operating system. Such devices are available fromMotorola and other IC producers. One of ordinary skill in the art willrecognize that a wide variety of microprocessors can be used based onthe speed and processing power desired for the given application. One ofordinary skill in the art will also recognize that processor 300 can beimplemented using an analog computer.

There are certain core functions that processor 300 must perform.Processor 300 receives and processes switching commands from the networkinterface and in turn relays the switch status to network control formonitoring purposes. Processor 300 controls the timing for eachswitching cell in LC device 200. As discussed above, processor 300controls the voltage modulation for each cell by using the temperaturevalues latched in A/D 302, along with the wavelength assigned to eachcell, to select the LUT values used to drive modulator 306. As part ofthe monitoring function discussed above, processor 300 monitors theperformance of device 200 by checking the extinction ratio. This isdiscussed in more detail below. As LC device 200 ages, the LUT valuesmay need to be adjusted. Using its powerful processing capabilities anda real time operating system, processor 300 can repopulate LUT 304periodically to maintain the performance characteristics of switch 10.

Voltage modulator 306 may be of any suitable type, but there is shown byway of example a pulse amplitude modulator (PAM) that provides theroot-mean-square value of voltage necessary to drive liquid crystalcells 20. A pulse width modulator (PWM) can also be used. One ofordinary skill in the art will recognize that any modulator capable ofdelivering RMS bias voltages in the off-state to the LC cells 20 in LCdevice 200 can be used. A square wave, a sinusoidal wave or a bipolarpulse width modulated signal (PWM) may be used to drive liquid crystalcells 20. Modulator 306 must also provide the on-state switching voltagerequired to toggle each liquid crystal cell 20 in device 200 between anoff-switch state and an on-switch state. Typically, the on-state voltageis approximately 10 volts and is independent of temperature andwavelength.

Liquid crystal device 200 may be of any suitable well known type, butthere is shown by way of example, an array of twisted nematic liquidcrystal cells 20. Array 200 is designed in accordance with thediscussion presented above with respect to FIGS. 1-9. One of ordinaryskill in the art will recognize that LC device 200 can also befabricated using super twisted nematic cells. One of ordinary skill inthe art will also recognize that LC device 200 may be implemented as a1×N array or an M×N array of LC cells 20 depending on the complexity ofswitch 10. Thus, in a 1×N array, switch 10 supports a single network andan M×N array supports M-network layers. Each cell 20 in the arrayoperates as a wavelength channel switch. Thus, each cell 20 in LC device200 is used to rotate or not rotate the polarization state of asingle-wavelength light signal depending on the switch state of the cell20. As discussed above, the switch state of each cell 20 is controlledby processor 300. The bandwidth of these channels can be either 50 Ghzor 100 Ghz. As discussed above, without compensation, an LC cell 20rotates a light signal by almost 90° when no power is applied. Byappropriately designing LC device 200, each cell 20 can be tuned to aminimum condition in the off state by applying a small bias voltage.

Optical switch 10 operates as follows. Processor 300 controls theswitching states of each cell 20 in LC device 200 according to networkcommand. Voltage modulator 306 has an individually controllableconnection to each cell 20 in LC device 200. Processor 300 supplies eachcontrollable connection with a value from LUT 304 to drive eachcontrollable connection in accordance with a network command.

As described above, the polarization state of an incident light signalis rotated in an “off” state. For each cell 20 in this switch state,processor 300 provides modulator 306 with the unique bias voltage valueand each of these cells are driven to a minimum condition by perfectingthe helical twist of the liquid crystal molecules in each of the cellsto a near perfect 90°. Thus, the extinction ratio of each cell is drivenover 40 dB. As discussed above, the bias voltages are typically lessthan 1.6 V_(RMS) and are a function of both temperature and wavelength.One of ordinary skill in the art will recognize that these values can bechanged dynamically by processor 300 as changes in the ambienttemperature of switch 10 are detected. These values can also be changedif the wavelength assignments of the cells are changed.

For those channels that are not switched, processor 300 causes thevoltage modulator 306 to apply an “on-state” voltage. The on-statevoltage must be at least 6V to remove the helical twist formed by theliquid crystal molecules in the quiescent state. In this state, thecells transmit the incident signal without rotating the polarizationstate.

In another embodiment of the invention, as embodied herein and as shownin FIG. 11, a wavelength selective cross-connect (WSXC) 10 is disclosed.Input fiber 12 and add fiber 16 are connected to input device 40 whichincludes collimators and beam separators. Input fiber 12 and add fiber16 carry a multiplexed stream of wavelength channels that are collimatedand split into their respective polarization components by device 40.The polarized signals are directed to demultiplexer 50 and separatedinto wavelength channels. Input birefringent optical system 52 directseach wavelength channel to its respective LC cell 20. As discussed abovewith respect to the first embodiment, electronic controller 30 controlseach LC cell 20 individually to drive them into the minimum conditionwhen in the off-state. The polarization state of the incident polarizedbeam is rotated by 90°. The cells 20 that are turned on do not rotatethe light propagating through the channel. They are supplied withapproximately 10 V rms, as described above. Output birefringent opticalsystem 62 spatially displaces the channels that are rotated by LC cell20. Birefringent optical system 62 does not spatially displace thewavelength channels that are not rotated. Both the spatially displacedchannels and the non-displaced channels are multiplexed by multiplexer60 into an output beam 600 and a drop beam 602. Output device 42collimates and combines the beams, and directs them into the outputfiber 14 and the drop fiber 18, respectively. Thus, each channel oninput fiber 12 and add fiber 16 can be cross-connected to either outputfiber 14 or drop fiber 18 with an intra-channel cross-talk below 40 dB.

As discussed above, one of ordinary skill in the art will also recognizethat LC device 200 may be implemented as a 1×N array or an M×N array ofLC cells 20 depending on the complexity of switch 10. In this alternateembodiment, input fiber 12 is replaced by M input fibers, where M is aninteger. Output fiber 14 is replaced by M output fiber. Thus, each ofthe M input, M output fibers, M add fibers, and M drop fibers wouldcorrespond to one of the M-rows of LC matrix 200. Although not shown inFIG. 11, it is contemplated that any of the drop ports for any given rowcan be routed and fedback to any of the add ports for a given row. Thiswould provide cross-connectivity between any of the M×N channels.

As embodied herein and depicted in FIG. 12, another alternativeembodiment of the invention is disclosed. In this embodiment,electronics controller 30 includes an optical monitor 80. Monitor 80includes monitor light source 82, polarizer 56, monitor LC channel 86,analyzer 66, and detector 84.

Monitor channel may be of any suitable type, but there is shown by wayof example an LC cell of the type described above. However, in thisembodiment the optical monitoring is provided using the “second” minimumof the LC cell 86. This significantly lowers the required wavelength formonitoring and thus lowers the cost of the optical source. For a 90° TNLC cell sandwiched between parallel polarizer, the extinction ratio isgiven by equations (1), (2) and (3), as discussed above. As discussed inequation (3), the values of integer m refer to the minimum conditionsand must be closely maintained to achieve the low crosstalk needed inthe WSXC. For instance, m=1 is the first minimum and m=2 is the secondminimum. By maintaining high contrast on the monitor channel, highisolation can be maintained on the signal channels.

Light source 82 may be of any suitable type, but there is shown by wayof example a laser diode that provides a light signal having a narrowline width. One of ordinary skill in the art will recognize the lightsource can be also constructed using an LED and a bandpass filter. Indetermining the required wavelength of the light source 82, one of thewavelength channels is used as a reference wavelength. In FIG. 12, λ₁ isdepicted as the reference wavelength. One of ordinary skill in the artwill recognize that any of the channel wavelengths can be used as thereference wavelength. As an example, the required wavelength for amonitor light source given the reference (signal) channel conditions of:

Reference Channel Wavelength, λ₁=λ_(ref)=1550 nm

Birefringence, Δn_(ref)/Δn_(mon)=0.9, Δn_(ref)=0.1

If the monitor wavelength is situated at the first minimum, thanλ_(mon)=λ_(ref). However, if the monitor wavelength is situated at thesecond minimum, we know from equation 3 that $\begin{matrix}{{u_{ref} = {\sqrt{3} = {{\frac{2\quad d}{\lambda_{ref}}\left( {\Delta \quad n} \right)_{ref}\quad {in}\quad {which}\quad m} = 1}}}{u_{mon} = {\sqrt{15} = {{\frac{2\quad d}{\lambda_{mon}}\left( {\Delta \quad n} \right)_{mon}\quad {in}\quad {which}\quad m} = 2}}}} & (4)\end{matrix}$

Taking the ratio in equation 4, we can obtain the followingrelationship: $\begin{matrix}{\frac{\lambda_{mon}}{\lambda_{ref}} = {\sqrt{\frac{3}{15}}\left( \frac{\left( {\Delta \quad n} \right)_{mon}}{\left( {\Delta \quad n} \right)_{ref}} \right)}} & (5)\end{matrix}$

Thus, solving equation given the conditions above gives a monitorwavelength of λ_(mon)=770 nm. By providing feedback on contrast ratio atthe monitor wavelength of 770 nm, one is effectively monitoring thesignal channels. The remaining channels can be compensated through knownlook up tables or equations. For instance, the optimal compensationvoltage as a function wavelength is shown in the experimental plot inFIG. 7.

Monitor detector 84 may be of any suitable type, but there is shown byway of example a PIN diode. One of ordinary skill in the art willrecognize that an avalanche photodiode (APD) could also be used.However, PIN diodes are less expensive, less sensitive to temperature,and they require a lower reverse bias voltage than that of the APD.

In an alternate embodiment of the present invention, a monitor isinstalled at each pixel. In this embodiment, each liquid crystal cell 20is elongated. A monitor detector 84 is disposed above or below the lightpath of the wavelength channel.

Monitor 80 operates as follows. Light source 82 emits light at monitorwavelength λ_(mon). Polarizer 56 allows only one polarization componentof the monitor light to pass. The monitor polarized light is incident onmonitor channel 86. Channel 86 operates in the same way that the signalchannels operate. If it is “off” the light will be rotated by adiabaticfollowing. If it is “on” the light will not be rotated. Analyzer 66 isidentical to polarizer 56 and only allows one polarization component topass. Detector 84 measures the light exiting analyzer 66. One ofordinary skill in the art will recognize that if monitor channel 86fails to rotate the incident monitor light by a perfect 90° when voltagecompensation is applied, light will be incident on detector 84. Detectoris connected to Processor 300 (connection not shown). Thus, opticalmonitor 80 provides feedback to processor 300 in electronic controller30 (See FIG. 10). Processor 300 has the option of adjusting LUT 304 tochange bias voltages, or temperature compensation module 34 to changethe operating temperature. Processor 300 could adjust both of thesefactors.

The monitor 80 and control electronics 30 compensate for changes in LCdevice 20 due to aging. For example, extreme temperature cycles couldresult in a LC thickness variation causing the optimal temperature toshift. As a result, the compensation parameters needed to drive thevarious channels into a minimum condition must be adjusted. Monitor 80provides extinction ratio monitoring. Thus, any increase inintra-channel cross-talk will be detected immediately and correctivemeasures will be implemented to maintain the low intra-channelcross-talk. In doing so, processor 300 performs a minimization routine.Referring to FIG. 3, the minimum condition curve may have moved due toaging. Thus, processor 300 will vary the off voltage to thereby find theminimum condition. Using FIG. 5, for example, processor 300 will thencalculate the values for the remaining wavelength channels. If processor300 cannot find the minimum condition, it would then signal an alarmthat a catastrophic error has occurred.

One of ordinary skill in thart will recognize that the ability toreconfigure optical switch 10 as discussed above, provides for moresophisticated techniques of network diagnostics, maintenance and faulttolerance. With the monitoring function discussed above, the Networkmanagers can be apprised of the condition of every switch in theNetwork. If a given switch 10 is out of tolerance, optical switch 10 canbe reconfigured without taking it off line. The diagnostic function ofswitching and compensation electronics 30 also determines if the switch10 is in an illegal state, or is experiencing either a catastrophicfailure condition or merely a degraded performance condition. This typeof warning is invaluable in fault tolerant switched rings or meshednetworks. In the case where a catastrophic failure condition occurs,traffic is rerouted around the failed switch. In addition, the generatedalarm is also used to issue a repair and/or replace order to maintenancepersonnel.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. An optical device for directing a light signalcomprising: a liquid crystal element for modulating the light signal,wherein said liquid crystal element is characterized by an extinctionratio when in an off-state; and a voltage controller connected to saidliquid crystal element for supplying a bias voltage to said liquidcrystal element in said off-state to drive said extinction ratio towarda minimum condition.
 2. The optical device of claim 1, wherein thevoltage controller varies the bias voltage in accordance with an ambienttemperature of the optical device and a wavelength of the light signalto thereby drive the extinction ratio to the minimum condition.
 3. Theoptical device of claim 2, wherein the liquid crystal element has athickness “d” which causes the minimum condition to occur at awavelength greater than the longest operating wavelength of the lightsignal and at a temperature greater than the maximum operatingtemperature of the optical device.
 4. The optical device of claim 3,wherein the extinction ratio is defined as:${\eta = \frac{\sin^{2}\left( {\frac{\pi}{2}\sqrt{1 + u^{2}}} \right)}{1 + u^{2}}},\quad {{and}\quad {the}\quad {minimum}\quad {condition}\quad {occurs}\quad {when}}$

and the minimym condition occurs when${{\frac{\pi}{2}\sqrt{1 + u^{2}}} = {m\quad \pi}},$

for m, an integer, wherein, for a given wavelength λ and for abirefringence Δn of said liquid crystal element, u is given by:$u = {2\frac{d}{\lambda}\Delta \quad n}$

where d is said thickness of said liquid crystal element.
 5. The opticaldevice of claim 1, wherein the extinction ratio is greater than 40 dB.6. The optical device of claim 1, wherein the extinction ratio isgreater than 50 dB.
 7. The optical device of claim 1, wherein the liquidcrystal element is a matrix of liquid crystal cells, and each of saidliquid crystal cells modulates a respective one of a plurality of signalchannels.
 8. The optical device of claim 7, wherein the light signalincludes a plurality of wavelengths, each of said plurality ofwavelengths corresponds to one of the signal channels.
 9. The opticaldevice of claim 8, wherein the voltage controller further comprises: avoltage modulator connected to the matrix of liquid crystal cells,wherein said voltage modulator includes an independently controllableconnection to each of the liquid crystal cells to thereby provide a cellbias voltage to drive the liquid crystal cell into a minimum condition;a look-up-table connected to said voltage modulator for supplying biasvoltage values for each independently controllable connection, whereinsaid look-up-table stores bias voltage values as a function ofwavelength and temperature for each wavelength and over an operatingrange of temperatures; a temperature sensor connected to the opticaldevice for measuring an ambient temperature of the optical device; and aprocessor connected to said temperature sensor, said look-up-table andsaid voltage modulator, said processor continually monitors said ambienttemperature and transfers said bias voltage values from saidlook-up-table to said voltage modulator.
 10. The optical device of claim9, wherein the minimum condition is an extinction ratio greater than 40dB.
 11. The optical device of claim 10, wherein the extinction ratio isgreater than 50 dB.
 12. The optical device of claim 9, furthercomprising a channel monitor for monitoring the extinction ratio of theliquid crystal element, said channel monitor including: a monitorswitching cell in the matrix of liquid crystal switching cells; amonitor light source coupled to said monitor switching cell on a firstside of said monitor switching cell, wherein said monitor light sourcepropagates a monitor light signal through said monitor switching cell;and a detector coupled to a second side of the monitor switching cell,for detecting the monitor light signal and producing a monitor signal.13. The optical device of claim 12, wherein the processor uses themonitor signal to determine an extinction ratio of the channel monitor.14. The optical device of claim 12, wherein the processor uses themonitor signal to determine whether the voltage bias values correspondto the minimum condition.
 15. The optical device of claim 14, whereinthe processor adjusts the voltage bias values in the look-up-table inaccordance with the monitor signal.
 16. The optical device of claim 12,further comprising a temperature compensation module connected to theprocessor, wherein said temperature compensation module regulates theambient temperature of the optical device under processor supervision.17. The optical device of claim 16, wherein the processor uses themonitor signal when adjusting the ambient temperature of the opticaldevice.
 18. The optical device of claim 9, wherein the cell bias voltageis a root-mean-square voltage value.
 19. The optical device of claim 18,wherein the cell bias voltage is an alternating current voltage.
 20. Theoptical device of claim 19, wherein the cell bias voltage is a pulseamplitude modulated signal.
 21. The optical device of claim 19, whereinthe cell bias voltage is a square wave.
 22. The optical device of claim19, wherein the cell bias voltage is a sinusoidal wave.
 23. The opticaldevice of claim 19, wherein the cell bias voltage is a pulse widthmodulated signal.
 24. The optical device of claim 8, further comprising:at least one input port for receiving the light signal from a network;at least one output port for transmitting an output light signal to saidnetwork; an input birefringent optical system connected to said at leastone input port and the matrix of liquid crystal cells, said inputbirefringent optical system converts the light signal into the pluralityof signal channels, wherein each of the liquid crystal cells switchesone signal channel in accordance with a cell switch state to produce asignal channel output; and an output birefringent optical systemconnected to the matrix of liquid crystal cells, said outputbirefringent optical system multiplexes said signal channel outputs intoeither an orthogonal polarized output component or a parallel polarizedoutput component based on said cell switch state, and directs saidorthogonal polarized output component and said parallel polarized outputcomponent into said at least one output port in accordance with thepolarization state of said orthogonal polarized output component andsaid parallel polarized output component.
 25. The optical device ofclaim 24, wherein the input birefringent optical system splits the lightsignal into an orthogonal polarized component and a parallel polarizedcomponent, and demultiplexes said orthogonal polarized component andsaid parallel polarized component into the plurality of wavelengths. 26.The optical device of claim 25, wherein the at least one input portincludes an input port and an add port, said add port includingwavelength channels that are interleaved with the plurality of signalchannels by the liquid crystal element and the input birefringentoptical system.
 27. The optical device of claim 24, wherein the at leastone output port includes an output port and a drop port, said drop portincluding wavelength channels that are dropped from the plurality ofsignal channels by the liquid crystal element and the outputbirefringent optical system.
 28. The optical device of claim 24, whereinthe at least one input port includes N input ports, and the at least oneoutput port includes N output ports, wherein N is an integer.
 29. Theoptical device of claim 28, wherein the matrix is an N×M matrix, whereinM is an integer number of signal channels.
 30. The optical device ofclaim 1, wherein the light signal is switched in accordance with apolarization state of the light signal.
 31. The optical device of claim30, wherein the bias voltage causes the liquid crystal element to rotatethe polarization state of the light signal by a degree of rotationsubstantially equal to 90°.
 32. The optical device of claim 31, whereinthe liquid crystal element rotates the polarization state of the lightsignal by a degree of rotation not equal to 90° when no bias voltage isapplied.
 33. The optical device of claim 30, wherein the voltagecontroller supplies the liquid crystal element with an on-state voltagesuch that the liquid crystal element does not rotate the polarizationstate of the light signal.
 34. The optical device of claim 33, whereinthe on-state voltage is not a function of a wavelength of the lightsignal or a temperature of the optical device.
 35. The optical device ofclaim 1, further comprising a channel monitor for monitoring theextinction ratio of the liquid crystal element, said channel monitorincluding: a monitor light source disposed on a first side of the liquidcrystal element, for directing a monitor light signal through the liquidcrystal element; a detector disposed on a second side of the liquidcrystal element, for detecting the monitor light signal as the monitorlight signal exits the liquid crystal element, wherein said detectorproduces a monitor output signal; and a processor connected to saiddetector, wherein said processor uses the monitor output signal todetect changes in at least one characteristic of the liquid crystalelement.
 36. The optical device of claim 35, wherein the processordrives the voltage controller and supplies the voltage controller with apredetermined bias voltage in accordance with the at least onecharacteristic, a wavelength of the light signal, and ambienttemperature.
 37. The optical device of claim 36, wherein the processoradjusts the bias voltage in response to detecting changes in the atleast one characteristic of the liquid crystal element.
 38. The opticaldevice of claim 37, wherein the at least one characteristic includes athickness variation of the liquid crystal element.
 39. The opticaldevice of claim 37, wherein the at least one characteristic includes arelationship between the extinction ratio, temperature and wavelength,due to aging of the liquid crystal element.
 40. The optical device ofclaim 35, wherein the channel monitor further comprises: a temperaturesensor for monitoring an ambient temperature of the optical device; anda temperature compensation module connected to the processor, whereinsaid temperature compensation module regulates said ambient temperatureof the optical device under processor supervision.
 41. A method ofdirecting a light signal in an optical device, said optical deviceincluding a liquid crystal element for switching the light signal, saidliquid crystal element being characterized by an extinction ratio whenin an off-state, said method comprising the steps of: providing theliquid crystal element with a thickness that causes a minimum conditionto occur at a maximum wavelength greater than the longest operatingwavelength of the light signal and at a maximum temperature greater thana maximum operating temperature of the optical device; and supplying abias voltage to the liquid crystal element in the off-state to drive theextinction ratio toward a minimum condition, wherein the wavelength ofthe light signal is smaller than the maximum wavelength and the ambienttemperature is less than the maximum temperature.
 42. The method ofclaim 41, wherein the liquid crystal element is a matrix of individuallycontrollable liquid crystal cells for individually switching singlewavelengths of the light signal.
 43. The method of claim 42, wherein thelight signal includes a plurality of wavelengths, and each of saidplurality of wavelengths corresponds to one of the individuallycontrollable liquid crystal cells.
 44. The method of claim 42, themethod further comprising the steps of: measuring an ambient temperatureof the optical device; providing a look-up-table connected to saidvoltage modulator for supplying cell bias voltage values for each liquidcrystal cell, wherein said look-up-table store bias voltage values as afunction of wavelength and said ambient temperature; and providing acell bias voltage to drive the liquid crystal cell into a minimumcondition, wherein said cell bias voltage corresponds to the wavelengthof the liquid crystal cell and said ambient temperature.
 45. The methodof claim 42, further comprising the steps of: directing a monitor lightsignal through a monitor liquid crystal cell, wherein said monitorliquid crystal cell is one of the individually controllable liquidcrystal cells; directing a monitor light signal through a monitor liquidcrystal cell, wherein said monitor liquid crystal cell is one of theindividually controllable liquid crystal cells; detecting the monitorlight signal as the monitor light signal exits the monitor liquidcrystal cell to thereby produce a monitor output signal; and determiningthe extinction ratio of the monitor liquid crystal cell from the monitoroutput signal.
 46. The method of claim 45, further comprising the stepsof: evaluating the monitor output signal to detect changes in at leastone characteristic of the liquid crystal element when the extinctionratio of the monitor output signal exceeds a predetermined level; andadjusting the bias voltage values in the look-up-table in accordancewith said detected changes.
 47. The method of claim 46, wherein the stepof adjusting further comprises the steps of: adjusting the cell biasvoltage of monitor liquid crystal cell to find a minimum condition; andadjusting all of the individually controllable liquid crystal cells inaccordance with said minimum condition.
 48. The method of claim 47,wherein the step of adjusting further comprises a step of signalling acatastrophic failure condition when the monitor liquid crystal cellcannot be adjusted into the minimum condition.
 49. A method offabricating an optical device for directing a light signal, said methodcomprising the steps of: providing a liquid crystal element forswitching the light signal, said liquid crystal element has a thickness“d” which causes a minimum condition of an extinction ratio to occur ata wavelength greater than a longest operating wavelength of the lightsignal and at a temperature greater than a maximum operating temperatureof the optical device; and providing a voltage controller connected tosaid liquid crystal element, wherein said voltage controller supplies abias voltage to said liquid crystal element in said off-state to drivesaid extinction ratio toward said minimum condition for operatingwavelengths shorter than said longest operating wavelength, and fortemperatures lower than said maximum operating temperature.
 50. Themethod of fabricating an optical device of claim 49, wherein theextinction ratio is defined as:${\eta = \frac{\sin^{2}\left( {\frac{\pi}{2}\sqrt{1 + u^{2}}} \right)}{1 + u^{2}}},\quad {{and}\quad {the}\quad {minimum}\quad {condition}\quad {occurs}\quad {when}}$

and the minimum condition occurs when${{\frac{\pi}{2}\sqrt{1 + u^{2}}} = {m\quad \pi}},$

for m, an integer, herein, for a given wavelength λ and for abirefringence Δn of said liquid crystal element, u is given by:$u = {2\frac{d}{\lambda}\Delta \quad n}$

where d is said thickness of said liquid crystal element.
 51. The methodof fabricating an optical device of claim 49, wherein the step ofproviding a liquid crystal cell further comprises providing multipleliquid crystal cells arranged to form a matrix of liquid crystal cells,and wherein the step of providing a voltage controller further comprisesthe steps of: providing a voltage modulator connected to the matrix ofliquid crystal cells, wherein said voltage modulator includes anindependently controllable connection to each of the liquid crystalcells to thereby provide a cell bias voltage to drive the liquid crystalcell into a minimum condition; providing a look-up-table connected tosaid voltage modulator for supplying bias voltage values for eachindependently controllable connection, wherein said providing aprocessor connected to said temperature sensor, said look-up-table andsaid voltage modulator, said processor continually monitors said ambienttemperature and transfers said bias voltage values from saidlook-up-table to said voltage modulator.
 52. The method of fabricatingan optical device of claim 49, further comprising the step of providinga channel monitor for monitoring the extinction ratio of the liquidcrystal element.
 53. The method of fabricating an optical device ofclaim 52, wherein the step of providing a channel monitor furthercomprises the steps of: providing a monitor light source disposed on afirst side of the liquid crystal element, for directing a monitor lightsignal through the liquid crystal element; providing a detector disposedon a second side of the liquid crystal element, for detecting themonitor light signal as the monitor light signal exits the liquidcrystal element, wherein said detector produces a monitor output signal;and providing a processor connected to said detector, wherein saidprocessor uses the monitor output signal to detect changes in at leastone characteristic of the liquid crystal element.
 54. The method offabricating an optical device of claim 49, further comprising the stepsof: providing a temperature sensor for monitoring an ambient temperatureof the optical device; and providing a temperature compensation moduleconnected to a processor, wherein said temperature compensation moduleregulates said ambient temperature of the optical device under processorsupervision.
 55. The method of fabricating the optical device of claim49 wherein the step of providing a liquid crystal cell further comprisesproviding multiple liquid crystal cells arranged to form a matrix ofliquid crystal cells, the method further comprising the steps of:providing at least one input port for receiving the light signal from anetwork; providing at least one output port for transmitting an outputlight signal to said network; providing an input birefringent opticalsystem connected to said at least one input port and the matrix ofliquid crystal cells, said input birefringent optical system convertsthe light signal into a plurality of signal channels, wherein each ofthe liquid providing at least one output port for transmitting an outputlight signal to said network; providing an input birefringent opticalsystem connected to said at least one input port and the matrix ofliquid crystal cells, said input birefringent optical system convertsthe light signal into a plurality of signal channels, wherein each ofthe liquid crystal switch cells switches one signal channel inaccordance with a cell switch state to produce a signal channel output;and providing an output birefringent optical system connected to thematrix of liquid crystal cells, said output birefringement opticalsystem multiplexes said signal channel outputs into either an orthogonalpolarized output component or a parallel polarized output componentbased on said cell switch state, and directs said orthogonal polarizedoutput component and said parallel polarized output component into saidat least one output port in accordance with the polarization state ofsaid orthogonal polarized output component and said parallel polarizedoutput component.
 56. The method of fabricating the optical device ofclaim 55, wherein the input birefringent optical system splits the lightsignal into an orthogonal polarized component and a parallel polarizedcomponent, and demultiplexes said orthogonal polarized component andsaid parallel polarized component into the plurality of wavelengths. 57.The method of fabricating the optical device of claim 55, wherein the atleast one input port includes an input port and an add port.
 58. Themethod of fabricating the optical device of claim 55, wherein the atleast one output port includes an output port and a drop port.
 59. Themethod of fabricating the optical device of claim 55, wherein the atleast one inport port includes N input ports, and the at least oneoutput port includes N output ports, wherein N is an integer.
 60. Themethod of fabricating the optical device of claim 59, wherein theoptical device is adapted for use with N input fibers each having Msignal channels where N and M are integers, and wherein the matrix is anN×M matrix.
 61. An optical device for directing a light signalcomprising: a liquid crystal element for modulating the light signal,wherein said liquid crystal element is characterized by an extinctionratio when in an off-state; and a controller connected to said liquidcrystal element for adjusting at least one parameter of said liquidcrystal element in said off-state to drive said extinction ratio towarda minimum condition.
 62. The optical device of claim 61, wherein the atleast one parameter is a bias voltage.
 63. The optical device of claim62, wherein the at least one parameter is an ambient temperature.